Treating a wood pulp slurry with a water-soluble acid prior to addition of sizing materials thereto



United States Patent TREATING A woof) PiJLP SLURRY WITH A WATER-SOLUBLE ACID PRIOR TO ADDI- TION 0F SIZING MATERIALS THERETO Harold L. Jones, Mobile, Ala., assignor to International Paper Company, New York, N.Y., a corporation of New York No Drawing. Filed May 10, 1960, Ser. No. 27,999

9 Claims. (Cl. 162179) This invention relates to sizing. More particularly, it relates to an unexpectedly improved method of preparing wood pulp for internal sizing.

The term sizing means many things to many people. In the textile industry, it is employed in relation to the dressing and preparation of textiles for printing and, in both the textile and paper industries, it refers to materials used to prevent water or ink absorption due to capillary attraction. Grant, J. L.; H-ackhs Chemical Dictionary (Third Edition); McGraw-Hill Book Company, Inc.; New York, 1944; p. 776. Again, in the paper industry, internal sizing is used to denote the addition of materials to paper stock prior to the formation of the sheet to impart water resistance to the finished paper or paperboard, e.g., to control the spreading or feathering of writing ink, while surface sizing is used to denote the treatment of the sheet in the dryer section of the paper machine to prevent undue penetration of ink and to produce a firm, hard surface for easy writing with pen and ink. Calkin, J. B.; Modern Pulp and Paper Making; Reinhold Publishing Corporation; New York, 1957; pp. 286 and 355.

No. 165, pp. 184 to 192 and 330 to 40s.

It is well-known that, [while in the surface sizing of paper, the most common sizing agents or sizes are a modified starch, a glue, or a mixture of the two, internal sizing can be and most often is accomplished merely by adding a rosin soap or an aqueous dispersion of rosin to the paper pulp slurry and setting it by adding aluminum sulphate (papermakers alum). Wax dispersions and fatty acid soaps on the one hand and aluminum chloride, sodium 'aluminate, or sodium phospho-aluminate on the other hand are also known in this regard. However, it is just as well known that it is highly desirable to keep the addition of sizing chemicals to a minimum. Not only can chemical costs not be ignored, but it must be considered that agents added for imparting Water resistance generally detract from paper strength. Ca-lkin, op. cit. supra at 286, 293 and 355. For this reason, it is not particularly surprising that research in sizes has touched such various things as aliphatic amides, black liquor, casein, cellulose derivatives, chitin, gelatinized cellulose and natural wood resin, humic acids, karaya gum, naphthinic acids, rubber latex, seaweed and its components, silicon compounds, soaps, sodium silicate, sulphite waste liquor, synthetic and natural resins, vegetable proteins, viscose wax sizing, wood tars, etc., and that research in alun substitutes has dealt with ferrous sulfate, niter cake, sodium acid sulfate, sulfuric acid, magnesium sulfate, zinc sulfate, chromic sulfate, ferric sulfate,-carbon dioxide, sulfur dioxide, aluminum chloride, sodium aluminate, aluminum silicate, magnesium chloride, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, ammonium nitrate, sodium nitrate, potassium nitrate, calcium nitrate, ammonium sulfate, potassium sulfate, cal- 3,112,242 Patented Nov. 26, 1963 cium sulfate, hydrochloric acid, copper sulfate, and aluminum phosphate sulfate. Sizing of Paper, The Institute of Paper Chemistry Bibliographic Series, Appleton, 1945, Somewhat more noteworthy is the fact that the use of sulfuric acid and hydrochloric acid as alum substitutes, for instance, has not proved commercially feasible.

Side by side with the efforts of those skilled in the art of the internal sizing of paper to find practical substitutes for the rosins and aluminum sulfate and the like have been their efforts to learn more of the basic mechanism of rosin sizing and, therefrom, of how to get more effect from lesser quantities of the still commonly employed sizing materials. It is in this latter area that the present invention affords a contribution to the art.

On the one hand, paper technologists have generally come to accept a colloidal theory of the mechanism of rosin sizing first presented by Lorenz in 1923 and that, while cellulose fibers and rosin are both negatively charged in water suspensions, aluminum ions are positively charged, so that it is believed that alumina binds rosin to the fibers by electrostatic forces. Calkin, op. cit. supra at 29 2 and 293. On the other hand, while what they usefully do may be clear, just precisely what these forces are and what they involve seems still to be under discussion, if not in controversy. Thus, for example, Cobb and Lowe introduce their concept of the mechanism of sizing with rosin, i.e., sizing with rosin acids such as abietic acid (C H COOH) in the saponified form, by reviewing the coordination theory of valency and they take the position that the most successful sizing arrangement is one in which the size, e.g., the abietate ion, and cellulose are coordinately bound to the aluminum ion; that the structure of cellulose with its three hydroxyl groups per molecule assures high coordinating potential; and, that abietic acid, notwithstanding its complexity, contains only one carboxyl group per molecule, so that it has a low coordinating potential. Tappi 38, No. 2:49 (1955). By so doing, they direct attention to the rosin size and to its Weaknesses in providing the desired linkages. Thode et al., however, direct attention to another point, i.e., the need for a sufficiently negative charge on the cellulose fibers to be sized to give rise to an attnaction forpreviously formed, positively charged, insoluble complexes of rosin and hydrated aluminum ions. Tappi 38, No. 12:270 (1955).

The discovery upon which the present invention is based was not made in an attempt to prove either Cobb and .Lowe or Thode et al. right and it does not in fact do so. It is broader in scope and practical effect than the discussion of either group in that it actually yields rosin sizing of heightened level for a given amount of sizing materials or of even quality with that heretofore available only by using greater amounts of such materials and it entails the use of means suggested by neither group. However, to some extent, the nature of the discovery is possibly illuminated by the discussions of both.

The present invention is grounded in the discovery that, when an aqueous slurry of wood pulp is treated in a first, pre-sizing step by the addition to it in the absence of sizing materials of a water-soluble acid and is then washed with water and sized or just sized, whether with a soap of a rosin acid, a rosin acid derivative, or a fatty acid or the acids themselves and whether with paper-makers alum (aluminum sulfate) or any other water-soluble salt of a trivalent metal, the sizing of the paper produced from such wood pulp is substantially improved. Nitric acid, hydrochloric acid, and sulfurous acid are among those preferred for use in the pre-sizing step, but sulfuric acid, acetic acid, citric acid, tartaric acid, and oxalic acid can be used too and especially if the water washing step is employed following the pre-sizing step.

It is believed that the process of the invention succeeds so remarkably well by eliminating from aqueous sizing solutions the deleterious effect of cations and anions which intrude upon and diminish the forces operating to tie the cellulose fibers, aluminum ions, and rosin together and the process will be discussed herein in such a light. However, the limitations of present theoretical knowledge should in no way be taken to limit the scope of the present invention.

In doing the research underlying the present discovery, it was considered that cotton linters and rag papers or any high alpha-cellulose papers are very difficult to size unless beaten long and hard, the coordination capacity of the cellulose structure posited by Cobb and Lowe to the contrary notwithstanding. It was further considered that the well beaten pulp appeared to be more attractive to the rosin size than did the unbeaten pulp as evidenced by the quality of the sizing achieved, perhaps for the reason suggested by Thode et al., i.e., the achievement in the beaten pulp of a sufficient negative charge on the cellulose fibers or, otherwise expressed, of more cellulose anions. As a consequence, the investigation turned to finding a way of assuring the presence of such cellulose anions in and the relative absence of deleterious cations capable of blocking such anions from uniting with aluminum and rosin in the usual aqueous sizing solutions. The use of a water-soluble acid, e.g., one displaying a higher degree of dissociation in water than the carboxylic acids in cellulose fibers giving rise to cellulose anions, came to mind. However, particularly because of the above-noted failures in attempts to employ acids as alum substitutes, the investigation had also to turn to finding out if such a step would adversely effect the aluminum-rosin complex by, for instance, giving it a charge insufficiently positive.

As noted by Cobb and Lowe, op. cit. supra, the aluminum ion combines coordinately with six molecules of water to form a complex ion [Al(H O) The bond by which the aluminum holds water molecules in the complex is one of coordinate covalence; the bond by which the complex is associated with outside ions-as in [Al(H O) ]Cl -is electrovalence; and, the covalence of aluminum is 6 and its electrovalence is 3. But, in addition, the water molecules that are bound to the central aluminum atom by covalence can be replaced by the abietate ion derived from one of the rosin acids or its soaps or the anion derived from fortified rosin (the reaction product of abietic acid and maleic or fumaric anhydride) or its soaps or can be replaced even more readily by other anions, so that the electrovalence can become 3, 2, l, or even a negative quantity, while the covalence stays at 6. Consequently, it seemed clear that the acid to be used to maintain the availability of cellulose anions could not contain anions which, because of their higher coordinating capacity, would replace the rosin anions in the aluminum-rosin complex or could not be used in such strength or quantity or at such a time that it would yield concentrations of ions of such an order that either the rosin anions would be blocked by sheer weight of numbers from combining with the aluminum or the few aluminum-rosin complexes which did form would be too negatively charged to combine properly with the cellulose anions. And the results of these considerations are reflected in the examples, infra, which make clear the importance of using the acid on the aqueous wood pulp to protect the cellulose anions, i.e., to demineralize the slurry, before the rosin size and the alum are added and, perhaps, so that the acid anions are placed in a poorer competitive position vis-a-vis rosin anions and cellulose anions in relation to the aluminum.

Example I Chlorine dioxide-bleached pulp was beaten to 550-freeness in a Valley beater at 1.75% consistency. The dilution water in the beater was demineralized water to which lime had been added to obtain a 7.0 pH. After beating, the pulp was dumped into a stainless steel container equipped with a slow speed stirrer. Eighteen hundred cubic centimeters of the pulp were then taken out of the container and diluted to 2000 cc. to give 550-freeness pulp in demineralized water at 1.57% consistency. Thereafter, wood rosin size (commercially available Accobrite N) was added to the pulp at a rate of 50 lbs. of size solids per ton of pulp and the slurry was stirred for five minutes with a Lightnin mixer. This was followed by the addition to the slurry of papermakers alum (aluminum sulfate) at the rate of 50 lbs. of alum per ton of pulp and further stirring for 10 minutes. The pH of the slurry was then adjusted to 5.5 with lime, stirred for 5 minutes, readjusted to 4.5 with alum, and again stirred for 5 minutes. Next, the slurry was diluted to a total of 5545 cc. (0.57% consistency) with demineralized water that had been treated with sufficient lime to give 35 ppm. CaCO and sufficient sulfuric acid to give pH of 4.5. It was stirred for 2 minutes. Using 800 cc. of the resulting slurry per sheet mould, 4 handsheets were made with subsequent pressing for 5 minutes at 50 p.s.i. All sheet mould dilution used the aforementioned treated demineralized water. After pressing, the sheets were clamped in rings and oven-dried (forced air at C.) for 15 minutes. On removing these sheets from the oven, they were placed in a polyethylene bag and transferred to a room maintained at 50% relative humidity. When these sheets were tested, they were shown to have an average basis weight of 152 1b., an average porosity of 139 sec./ 100 ml., an average caliper of 13.0 points. They gave a 20% lactic acid penescope test of 209 sec. and a wet break test of 372 sec. (one kilogram weight).

Example II Seventy-five grams (oven-dry basis) of refined, chlorine dioxide-bleached pulp was treated with 2500 ml. 0.1% hydrochloric acid (pH 1.5) overnight. By morning, the stock had a pH of 2.2. Next, the pulp was dewatered to 28.9% consistency and sized without removal of the residual acid, but by the technique set forth in Example I, except for the fact that the pulp was suspended in dilution water at pH 4.5 before the addition of the rosin size itself. Handsheets made from this sized pulp were made in accordance with the description of Example I and were found to have an average basis weight of 144 lbs., an average porosity of 142 see/100 ml., an average caliper of 12.4 points. They gave a 20% lactic acid penescope test of 2391 sec. and a wet break test of 612 sec. (one kilogram weight).

Example III The experiment of Example II was repeated, except that sulfurous acid was used rather than hydrochloric acid. It was noted that the acidity of the acid-treated pulp dropped to pH 2.4 in 16 hours. Handsheets made from such pulp were found to have an average basis weight of 151 lbs., an average porosity of sec./ 100 ml., an average caliper of 13.1 points. The 20% lactic acid penescope test was 1512 see. and the wet break test was 654 sec. (one kilogram weight).

Example IV The experiment of Example II was repeated, except that, in the morning, the pulp was filtered and washed with distilled water until its pH was brought to 4.5. Then, it was filtered again and suspended in water that had been adjusted to pH 4.5 with hydrochloric acid.

Rosin size and alum were added as before, the postside pH was adjusted to 4.5 with the smallest possible volume of a solution of sodium acetate. And, for the dilution to 0.57% consistency for handsheet preparation, water having a pH adjusted to 4.5 with hydrochloric acid was added to the sized pulp. Handsheets which resulted were found to have an average basis weight of 147 lbs., an average porosity of 99 sec./ 100 ml., an average caliper of 13.1 points. 20% lactic acid penescope time was 587 sec. and wet break time was 441 see. (one kilogram weight).

Example V The experiment of Example IV was repeated, except that sulfurous acid was used rather than hydrochloric acid. Handsheets produced from the pulp sized in such experiment were found to have an average basis weight of 154 lbs., an average porosity of 107 sec./ 100 ml., an average caliper of 13.4 points. The 20% lactic acid penescope test time was 526 sec. and the wet break test time was 702 see. (one kilogram weight).

Example VI As in Example II, the chlorine dioxide-bleached pulp was treated with 0.1% hydrochloric acid. However, in order to simulate actual mill conditions more accurately, the treatment was permitted to last only one hour, whereupon the pulp was filtered to approximately 20% consistency and was washed with water containing 35 ppm. CaCO and adjusted to pH 4.5 with sulfuric acid. The volume of the water used for washing was 1% times that present in the pulps when they were at 20% consistency. The pulp was then diluted to 3.0 to 3.5% consistency and sized as before with rosin size and alum. Lime was added to raise the pH to 6.0, and alum was added to trim the pH to 4.5. Hand-sheets were prepared immediately and were found to have an average basis weight of 148 lbs., an average porosity of 154 sec./ 100 ml., an average caliper of 12.7 points. 20% lactic acid penescope test time was 2143 sec. and the wet break test time was 578 sec. (one kilogram weight).

Example VII The experiment of Example VI was repeated, except that sulfurous acid was used in place of the hydrochloric acid. Handsheets produced by such method were found to have an average basis weight of 149 lbs., an average porosity of 132 sec./ 100 ml., an average caliper of 12.8 points. The 20% lactic acid penescope test time was 2709 sec.; the wet break test time was 892 see. (one kilogram weight).

Example VIII The experiment of Example VI was repeated, except that, when the lime was added to the pulp to give it a pH of 6.0, the pulp was allowed to hold such pH for one hour. Handsheets produced by such method were found to have an average basis weight of 149 lbs., an average porosity of 158 sec/100 ml., an average caliper of 12.9 points. The 20% lactic acid penescope test time was 2018 sec., and the wet break test time was 892 sec. (one kilogram weight).

Example IX The experiment of Example VII was repeated, except that, when the lime was added to the pulp to give it a pH of 6.0, the pulp was allowed to hold such pH for 1 hour. Handsheets produced by such method were found to have an average basis weight of 151 Has, an average porosity of 155 sec./ 100 ml., an average caliper of 12.8 points. The 20% lactic acid penescope test time was 2113 sec., and the wet break test time was 840 sec. (one kilogram weight).

Example X The experiment of Example I was repeated, except that the acid treated pulp was washed with distilled water to give the pulp a pH of 4.5 and filtered before the sizing.

Handsheets produced by these means were found to have an average basis weight of 148 lbs., and average porosity of 117 sec/ ml., a caliper of 12.9 points. The 20% lactic acid penescope test time was 4777 sec., and the wet break test time was 840 sec. (one kilogram weight).

Example XI A number of bleached pulp handsheets were sized with less than the standard quantities of rosin size (commercially available RP. 4027 containing 70% tall oil rosin and 30% gum rosin) and alum after the pulp was prepared as in Example II. The results were as follows:

l. A process comprising preparing wood pulp for sizing by adding to an aqueous slurry of the wood pulp and in the absence of sizing materials a water-soluble acid in an amount sufiicient to lower the pH of the slurry to 5 and below.

2. A process comprising preparing wood pulp for sizing by a first step of adding to an aqueous slurry of the wood pulp and in the absence of sizing materials a water-soluble acid, a second step of filtering the acid slurry, and a third step of washing the residue with water.

3. A process comprising preparing wood pulp for sizing by a first step of adding to an aqueous slurry of the wood pulp and in the absence of sizing materials a watersoluble acid in an amount sufiicient to lower the pH of the slurry to 5 and below, a second step of filtering the acid slurry, and a third step of washing the residue with water.

4. A process comprising a first step of adding to an aqueous slurry of wood pulp and in the absence of sizing materials a water-soluble acid in an amount sufiicient to lower the pH of the slurry to 5 and below and a second step of adding to the resulting slurry sizing materials including at least one member of the group consisting of fatty acids, fatty acid soaps, rosin acids, rosin acid soaps, rosin acid derivatives, and rosin acid derivative soaps and including a water-soluble salt of a trivalent metal.

5. A process comprising a first step of adding to an aqueous slurry of wood pulp and in the absence of sizing materials a water-soluble acid, a second step of filtering the acid slurry, a third step of washing the residue with water, a fourth step of slurrying the residue in water, and a fifth step of adding to the resulting slurry sizing materials including at least one member of the group con sisting of fatty acids, fatty acid soaps, rosin acids, rosin acid soaps, rosin acid derivatives, and rosin acid derivative soaps and including a water-soluble salt of a trivalent metal.

6. A process comprising a first step of adding to an aqueous slurry of wood pulp and in the absence of sizing materials a Water-soluble acid in an amount sufficient to lower the pH of the slurry to 5 and below, a second step of filtering the acid slurry, a third step of washing the residue with water, a fourth step of slurrying the residue in water, and a fifth step of adding to the resulting slurry sizing materials including at least one member selected from the group consisting of fatty acids, fatty acid soaps, rosin acids, rosin acid soaps, rosin acid derivatives, and rosin acid derivative soaps and including a water-soluble salt of a trivalent metal.

7. A process consisting essentially of a first step of adding to an aqueous slurry of wood pulp a water-soluble acid in an amount sufficient to lower the pH of the slurry 9. The process of claim 7 in which the acid is sulfurous to 5 and below, a second step of filtering the acid slurry, acid.

a third step of Washing the residue with water, a fourth step of slurrying the residue in water, and a fifth step of References Clted 111 the filfi 0f thls Patent adding to the resulting slurry sizing materials including at UNITED STATES PATENTS least one member selected from the group consisting of i 1,923,292 Bassett Aug. 22, 1933 fatty aclds, fat y acid soaps, I'OSII'I aCIdS, rosin aCld soaps, 2 041 285 De Cew M May 19 1936 rosin acid derivatives, and rosin acid derivative soaps and 2797163 Smith et a1. June 25 1957 including a Water-soluble salt of a trivalent metal.

8. The process of claim 7 in which the acid is hydro- 1O FOREIGN PATENTS chloric acid. 2,440 Great Britain 1854 

1. A PROCESS COMPRISING PREPARING WOOD PULP FOR SIZING BY ADDING TO AN AQUEOUS SLURRY OF THE WOOD PULP AND IN THE ABSENCE OF SIZING MATERIALS A WATER-SOLUBLE ACID IN AN AMOUNT SUFFICIENT TO LOWER THE PH OF THE SLURRY TO 5 AND BELOW. 